I have never heard of an atheist teaching his or her child to believe in god. Or, at least, not an open atheist; I imagine that there are atheists who pretend to be theists that are raising their children as such.

I think it's because they desperately want to believe in a god and if they teach someone else to, they may convince themselves at the same time. It sounds like these people are really in a state of transition or crisis regarding their belief system or identity.
All speculation though.

Since I don't have any children, I faced this "dilemma" with my little brother (8 years-old). Raised by my parents (liberal Catholics), he grew up believing in a creator. Before I could indulge in any discussion with him I had to convince my parents first, not to keep on encouraging him to believe in god. I couldn't persuade them to let me handle it and to change his outlook completely. However, I made them promise that they would no longer discuss religion with him. I also convinced that when he starts asking the usual questions (about creation, why we are here, etc..), which every kid will ask at one point during his or her childhood, they should not answer him; instead, I will try to communicate to him the best scientific answer there is, or I can simply tell him "no body knows, yet"!